The present invention relates generally to a rotation sensor which monitors the angular velocity of a rotating component, such as the rotary head of a video tape recorder and/or player. More particularly, the invention relates to a rotation sensor including a reference signal generator, which will be referred to hereafter as an "FG detector", producing a pulse for each unit of rotation of the rotary component and a position signal generator, which will be referred to hereafter as a "PG detector", producing a reference position indicative pulse at a predetermined reference position or positions of the rotary component relative to a stationary component.
In a motor driven rotary system such as the rotary head of a video tape recorder and/or player, which will be referred to hereafter as a "VTR head", it is necessary to control servo-monitors thereof to control angular velocity and phase synchronization. In the VTR head, angular velocity control is necessary for stability of head rotation and phase synchronization control is required to record the vertical hold control signal along one edge of the video tape. In order to perform both angular velocity control and phase synchronization control, a rotation sensor including a PG detector and a FG detector is employed in the servo system of the VTR head, which comprises a velocity control servo and a phase control servo.
FIGS. 1 and 2 show a typical conventional rotation sensor employing integral PG and FG detectors. FIG. 1 shows a stationary component comprising a printed conductor pattern 10 on a printed circuit board. FIG. 2 shows a rotary component comprising a magnetic ring 20, on the periphery of which are formed a plurality of blocks 22a, 22b, 24a and 24b of alternating N and S polarity. The printed circuit board serving as the stationary component is coaxially aligned with the magnetic ring 20 serving as the rotatable component with the printed conductor pattern 10 opposing the magnetic blocks 22a, 22b, 24a and 24b across a short gap. The printed conductor pattern 10 includes radially extending FG detector elements 13a. The angular pitch of each FG detector element 13a corresponds to a predetermined angle at which a reference signal S.sub.FG is to be produced, which is in the form of a sinusoidal wave as shown in FIG. 3. The FG detector elements 13a are connected to each other by outer peripheral elements 13b and inner peripheral elements 13c to form a complete FG detector circuit 13.
The angular width of magnetic blocks 22a and 22b corresponds to the angular pitch of the FG detector elements 13a. The magnetic blocks 22a and 22b cooperate with the FG detector elements 13a to act as a frequency generator outputting a sinusoidal reference signal pulse with each unit of rotation of the rotary component. This reference signal may be utilized for vertical hold control when the rotation sensor is employed in a VTR head servo system.
Each magnetic block 22a or 22b generates magnetic flux directed toward the adjacent magnetic blocks. The FG detector elements 13a cross the paths of the magnetic flux, which induces therein electric fields of a phase depending upon the instantaneous orientation of the magnetic flux. Therefore, as the magnetic ring 20 rotates, an alternating-current-type signal is induced in the FG detector 13. Since all of the FG detector elements 13a are connected to one another, the induced electricity is integrated over all of the elements and output as a single alternating-current-type signal via the FG output terminal 17 and a common terminal 16.
PG detector elements 14a also extend radially and have an angular pitch smaller than that of the FG detector elements 13a. Specifically, the pitch of the PG detector elements 14a is half that of the FG detector elements 13a. In addition, the phase of the PG detector elements 14a matches that of the FG detector elements. The magnetic blocks 24a and 24b respectively have the same angular spacing as the PG detector elements 14a. In the shown arrangement, the magnetic blocks 24a and 24b align with the PG detector elements once per cycle of rotation of the magnetic ring 20. The PG detector elements 14a cross magnetic flux paths formed between adjacent magnetic blocks 24a and 24b, 22a and 22b, and 24b and 22a, which induce electric fields interpreted as the position signal S.sub.PG. The PG detector elements 14a are respectively connected to one another by means of outer and inner peripheral elements 14b and 14c, and connected to the FG detector elements 13 via one of the inner peripheral elements 13c. The PG detector elements 14a are also connected to PG output terminal 15 and the common terminal 16 so as to output the position signal S.sub.PG once in every cycle of magnetic ring rotation. The waveform of the position signal S.sub.PG is illustrated in FIG. 3.
As the PG detector elements 14a cross the magnetic flux lines between adjacent magnetic blocks 22a and 22b, an equal number of induced fields of opposite polarity are always induced in the PG detector elements 14a so that the integrated signal strength will be zero. For instance, when the PG detectors 14a are aligned with an adjoining pair of the magnetic blocks 22a and 22b, each adjoining pair of PG detector elements 14a is aligned with a single magnetic element so that the signal components in the detector elements 14a of each pair cancel. On the other hand, when adjoining PG detector elements 14a are aligned with adjoining magnetic blocks 24a and 24b, the elements 14a cross anti-parallel magnetic flux lines so that the electrical signal components reinforce each other, resulting in a position signal pulse. As a result, the electricity induced in the PG detector by rotation of the magnetic ring remains nil until the magnetic blocks 24a and 24b move into alignment with the PG detector elements 14a.
The position signals S.sub.PG may serve as a head position indicative signal when the rotation sensor is employed in a VTR servo system.
When the rotation sensor is used in a VTR head servo system, the position signal phase must sometimes be shifted through 1.degree. or 2.degree.. Of course, the required signal phase shift can be achieved by means of a phase converter in a control circuit of the VTR system. Usually, however, this signal phase shift is performed by displacing the PG detector elements by the desired angle relative to the FG detector elements. However, this displacement of the PG detector elements results in incomplete cancellation of the noise components in the reference signals S.sub.FG induced as the PG detector elements cross magnetic flux lines established by the magnetic blocks 24a and 24b. Consequently, the magnetic blocks 24a and 24b adversely influence the reference signal once per rotation due to the serial connection of the PG detector in the FG detector circuit. This influence is predominantly AM interference in the FM reference.
The interference caused by the aforementioned influence of the offset of the PG detector elements is illustrated in FIG. 4 in the form of the wave forms resulting when the S.sub.FG carrier signal is checked by a wow-flutter meter. In the waveforms illustrated in FIG. 4, the waveform A represents the results of shifting the PG detector elements through 2.degree. with respect to the FG detector elements, the waveform B represents the results of shifting the PG detector element through 1.degree. relative to the FG detector elements; and the waveform C shows a typical wow-flutter trace when the PG detector elements are not offset. Theoretically, there will be no influence on the S.sub.FG carrier frequency when the PG detector elements are not offset relative to the FG detector elements. Negligibly small fluctuations in the S.sub.FG wow-flutter trace of the carrier may occur due to slight offsets among the FG detector elements and the corresponding magnetic blocks. When the PG detector elements are shifted 1.degree., the noise in the S.sub.FG carrier can be observed immediately after production of the position signal pulse. This effect on the S.sub.FG carrier becomes even greater when the phase shift is 2.degree..
When such a conventional rotation sensor is employed in a VTR servo system, the aforementioned fluctuations in the S.sub.FG carrier frequency may adversely affect speed control. As is well known, feedback control is performed on the basis of deviations in the S.sub.FG carrier frequency relative to a reference frequency to control acceleration and deceleration of the servo-motor. Therefore, fluctuations in the S.sub.FG carrier frequency due to the influence of the PG detector elements as described above cause matching fluctuations in the drive speed of the rotary head once per cycle of rotation. This causes "jitter" in the video signal, which degrades the reproduced image. In particular, servo systems in portable VTR sets have a relatively wide dynamic range in order to compensate for rolling and vibration of the set itself, and in such cases the influence of the PG detector element offset described above is much more serious.
Secondary interference in the S.sub.FG carrier frequency can be traced to the deviation between the electric fields induced by the PG magnetic blocks 24 in the radial conductors connecting the integrated FG detector elements to the common terminal 16 and the FG output terminal 17. Therefore, it appears that by avoiding the influence of the magnetic flux of the PG magnetic blocks 24 on these conductor lines, this noise component in the frequency signal can be eliminated.